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IAB = compression = Adiabatic expansion

BC isovohmetric changes

This expression suggests that to maximize the efficiency, (T, - TA)/(Tc - TB) must be as small as possible. This can be achieved by making the ratio of the maximum volume contained by the cylinder to the minimum volume contained by the cylinder (i.e. V,/V,) as large as possible. This is called the compression ratio, r. The relationship between the efficiency of the engine and the compression ratio is then given by:

where y is the adiabatic index or the ratio of the principal specific heat capacities (c,/c,), and y depends on the atomicity of the fluid used; r is effectively reduced if the ignition is inappropriately timed.